This invention pertains to a type of micro-electromechanical system. In particular, this invention pertains to a type of device or system which is required to have optical quality or reliability.
A micro-electromechanical system (MEMS) is usually a system that has electrically controllable micromachines (such as a motor, gear, optical modulating element, etc.) formed monolithically on a semiconductor substrate by means of photoetching or another wafer process.
A DMD (Digital Micromirror Device), a type of MEMS, is a reflective type optical modulator with electrical/mechanical/optical functions assembled on a single substrate. For example, plural (usually a fraction of a million or more) movable mirrors in a prescribed size (such as 16 xcexcm square) are set in matrix constitution with a prescribed pitch (say, 17 xcexcm) on a silicon substrate, and each movable mirror is set in one of two directions preset corresponding to the output of the corresponding address circuit.
In prior art, for a DMD for image display, improvement of the contrast ratio, that is, the contrast ratio of full ON (ON for all pixels)/full OFF (OFF for all pixels), is a topic to be addressed. In home entertainment, digital cinema, and other applications which require high image quality, a contrast ratio similar to that of a CRT (about 1000:1) is desired, yet the contrast ratio of a conventional DMD is at most about 500:1.
The contrast ratio of a DMD depends on the ratio of the light intensity from ON pixels to that from OFF pixels. Light leaking from OFF pixels becomes the principal factor in limiting the contrast ratio. This type of leaked light is mainly due to light that enters the mirror gap, is reflected by the lower structure and leaks from mirror gap and as well as light scattered and diffracted from supporting holes that support mirrors on yokes.
The objective of this invention is to solve the aforementioned problems of conventional methods by providing a type of micro-electromechanical system which can suppress undesired scattered and leaking light so as to have improved optical function and reliability.
Another objective of this invention is to provide a type of micro-electromechanical system which can reduce or prevent light reflection from the lower structure and upper structure of the movable portion so as to improve the optical function and reliability.
Yet another objective of this invention is to provide a type of micro-electromechanical system for image display with significantly improved contrast ratio.
In order to realize the aforementioned objectives, this invention provides a type of micro-electromechanical system characterized by the fact that in a micro-electromechanical system, which has an electrical circuit and a movable portion that can be driven and controlled by said electrical circuit on a substrate, there is an optical absorptive film that covers a portion or all of the members set on said substrate.
For the micro-electromechanical system of this invention, an optical absorptive film is coated on the portion of the substrate that would otherwise become a source of undesired leaking light or scattered light. Consequently, light is absorbed by the film, so that reflection and scattering of light can be alleviated or prevented. In particular, plural movable portions are formed, with said optical absorptive film formed in the regions below the gaps between the adjacent movable portions, so that leaking light from gaps can be prevented effectively. In addition, by also forming said optical absorptive film on holes formed on the surface of the movable portions, leaking light and scattered light from said holes can be reduced or prevented. Consequently, when movable portions form the optical element for image display, it is possible to improve the contrast ratio significantly.
The optical absorptive film in this invention is preferably made of an electrical insulator. In this case, even when plural electroconductive members are all covered with the film, there is still no electrical short circuit, and the film can be coated in a stable way. Also, when contact is made with the movable portions, the film should have an appropriate hardness so that it does not stick.
The optical absorptive film in this invention is preferably a fluorine-containing resin film having carbon black dispersed in it. More preferably, the film is a coating composition having necessary ingredients including fluorine-containing polymer (A) that has functional groups and a fluorine-containing resin cyclic structure in its principal chain, carbon black (B) with an average particle size smaller than 0.1 xcexcm, oligomer-type fluorine-containing surfactant (C), aprotic fluorine-containing solvent (D), and fluorine-containing alcohol (E). It is a fluorine-containing resin film with a composition with the following features: the proportion of carbon black (B) with respect to the sum of fluorine-containing polymer (A) and carbon black (B) is 1-30 mass %; the proportion of oligomer-type fluorine-containing surfactant (C) with respect to carbon black (B) is 1-20 mass %; and the proportion of fluorine-containing alcohol (E) with respect to the total mass of aprotic fluorine-containing solvent (D) and fluorine-containing alcohol (E) is in the range of 10-20 mass %.
When said composition is used, it is possible to form a black fluorine-containing resin coating with carbon black dispersed as primary particles with a size on the order of nm. The fluorine-containing resin formed from said composition exhibits good electrical insulation, low water absorptivity, and low refractive index based on the characteristics of the fluorine-containing resin, and it is possible to form a black coating film with very high optical reliability.